The present invention relates to an air-fuel ratio control system for an internal combustion engine mounted on a vehicle, which controls the air-fuel ratio of air-fuel mixture to a value approximately equal to the stoichiometric air-fuel ratio at which a three-way catalyst acts most effectively, and more particularly to an air-fuel ratio control system which is capable of improving the driveability of the vehicle during heavy load operation by holding the air-fuel ratio to a predetermined value.
In a conventional air-fuel ratio control system, the air-fuel ratio of the air-fuel mixture burned in the cylinders of the engine is detected as an oxygen density of the exhaust gases by means of an O.sub.2 sensor provided in the exhaust system of the engine. A determination is made by an output signal from the O.sub.2 sensor as to whether the air-fuel ratio is richer or leaner than the value corresponding to the stoichiometric air-fuel ratio for producing the control signal. The control signal is converted to pulses which operate an electromagnetic valve for regulating the feed rate of air to be mixed with the mixture. Thus, the air-fuel ratio is controlled to the stoichiometric air-fuel ratio at which three-way catalyst acts most effectively. In such an air-fuel ratio control system, when the throttle valve of the engine is opened wide or fully opened at heavy load, the feedback control operation, which depends on the detected signal by the O.sub.2 sensor, is stopped and the control signal is fixed, by an enriching system, to a predetermined value, so that the correction air rate is held to a predetermined value to enrich the air-fuel mixture so as to improve the driveability of the vehicle.
FIG. 5 shows such control ranges. A load detecting curve detected by a load sensor is in a lower position than a wide open throttle WOT curve. In the region below the load detecting curve, the feedback control operation is carried out, and in the region between the load detecting curve and the wide open throttle curve, the feedback control is not operated and the air-fuel ratio is fixed to a predetermined value.
In FIG. 5 reference Y represents an output torque curve relative to the engine speed when the vehicle is rapidly started. In such an operation, the output torque curve has a steep inclination. The output torque decreases in the fixed air-fuel ratio zone because of insufficient air-fuel ratio of the mixture. In order to resolve such a problem, if the air-fuel ratio is fixed to a small value, which means a rich air-fuel mixture, the mixture becomes excessively rich in a high engine speed zone resulting in decrease of the output of the engine.